This invention is directed to a method for preparing wet-collected iron-bearing fume and water-leached dry-collected iron-bearing fume and mixtures thereof for use in steelmaking furnaces to recover said iron. More specifically, the invention is directed to a method for treating wet or moist fume, which method includes drying, impact-fracturing and forming the fume into green balls on a balling device. The green balls have sufficient strength to resist degradation during the handling required to transport and charge them into a steelmaking furnace. Green balls may be defined as agglomerates which have been formed into balls or pellets and which have not been further subjected to a heat hardening. The green balls may be either moist or may have been dried, but will not have hardened at elevated temperatures such as would cause bonding of the component particles due to sintering together of the particles.
Fume formed in steelmaking furnaces during the refining of molten iron consists of fine particles of various elements and metallic and non-metallic compounds, such as iron oxides, zinc oxides, lead oxides, sulfur and sulfur compounds, carbon, silica, alumina, lime, magnesia and the like. These particles ae volatilized at the high temperatures within the steelmaking furnaces near the surface of the molten metal, condensed at lower temperatures and carried out of the furnaces by the furnace exhaust gases. The particles range in size from about one tenth of a micron to 25 microns or more in diameter. Usually more than about 90% of the particles will be less than 25 microns in diameter. The nature of fume from steelmaking furnaces is discussed in some detail in the following articles: (a) "Progress Review No. 63: The Formation and Suppression of Oxide Fume in Steelmaking"; Munro et al, Journal of the Institute of Fuel; March 1971, pp. 156-163, (b) "A New Look at the Cause of Fuming"; Morris et al, Jouranl of Metals; July 1966; pp. 803-810.
Because of their small size and weight, particles of fume are easily carried out of the furnaces through the furnace stacks to the atmosphere. Therefore, in order to prevent pollution of the atmosphere, the exhaust gases from steelmaking furnaces are customarily passed through air pollution control devices, for example wet scrubbers, dry electrostatic precipitators, bag-houses and the like which remove a substantial portion if not all of the fume from the exhaust gases. Since fume from steelmaking furnaces usually contains a relatively high percentage, for example over 50%, of iron in the form of iron oxides, it is economically desirable to recover the iron. It is possible to reuse the fume as charge material for steelmaking furnaces. However, because of the fine particle size of the collected fume, it is impractical to charge the fume, without some further treatment, into steelmaking furnaces, because the fume would merely be blown or carried out of the furnaces by the exhaust gases.
Wet-collected fume has in the past been agglomerated in order to increase the effective size of the particles. However, the moist agglomerates so formed do not possess sufficient strength to support a load and hence cannot be successfully transported, stored or charged into steelmaking furnaces without degradation of the agglomerated fume. Attempts to increase the strength of the moist agglomerates by low temperature drying have failed and in addition the dried agglomerates have spalled and cracked.
Moist fume can be balled and the green balls dried and then treated at elevated temperatures within the usual sintering or indurating ranges to produce heat-hardened pellets which do have sufficient strength to support a load and which can be transported and charged into steelmaking furnaces. However, the production of heat-hardened pellets is expensive because of the temperature involved and has not generally been commerically attractive; nor has it been accepted by the Steel industry.
Dry-collected fume from steelmaking furnaces, such as is frequently collected from open hearth furnace exhaust gases, can contain relatively high contents of sulfur, for example 0.30% by weight. Because of the relatively high sulfur content, the fume cannot be used as a charge material in steelmaking furnaces without preliminary treatment to lower the sulfur content. The dry-collected fume is usually treated by water leaching to reduce the sulfur content to acceptable levels, for example, under 0.20% by weight. The water leached dry-collected fume is, however, difficult to agglomerate. The agglomerates formed from water leached fume do not posses sufficient strength to support a load and cannot be transported, stored or charged into steelmaking furnaces without degradation of the agglomerates unless the agglomerates are also first treated at elevated temperatures in order to heat harden them.
There has been, therefore, a need for a simple, relatively inexpensive method for preparing moist, iron-bearing fume for use in steelmaking furnaces to recover the iron.
It is an object of this invention to provide a method for preparing moist iron-bearing fume for use in steelmaking furnaces, comprising substantially instantaneously drying the fume, impact-fracturing the dried fume and balling the impacted fume on a balling device to form green balls.
It is another object of this invention to provide a method for preparing high sulfur iron-bearing dry-collected fume for use in steelmaking furnaces, which method includes water leaching the dry-collected fume, substantially instantaneously drying the leached fume, impact-fracturing the dried fume and balling the fume on a balling device to form green balls.
It is another object of this invention to provide a method for preparing a mixture of high sulfur iron-bearing dry-collected fume and iron-bearing wet-collected fume for use in steelmaking furnaces. The method includes leaching the dry-collected fume, mixing the leached fume with the wet-collected fume, substantially instantaneously drying the mixed fumes, impact-fracturing the dried mixed fumes, and balling the dried mixed fumes on a balling device to form green balls.
It is still another object of this invention to produce green balls suitable for charging into steelmaking furnaces from iron-bearing wet-collected fume and mixtures of the wet-collected fume and high sulfur, iron-bearing leached dry-collected fume, said green balls being characterized by having the strength to survive at least 10 drops from a height of 18 inches in a drop test and having a compression strength of at least 15 pounds per square inch and which resist degradation during handling and transport.